1. Field of the Invention
The present invention relates to a pre-emphasis control method for use with a wavelength division multiplexing communications system.
2. Description of the Related Art
Recently, an optical communications network has been realized as a communications circuit. Especially, to realize a high-speed large-capacity data communications, a WDM (wavelength division multiplexing) communications system is the dominating optical communications. It is expected that larger capacity and higher speed communications circuits are demanded and the development of a system which allows such communications capacity is strongly desired.
Under the circumstances, a system for further increasing the number of multiplexed wavelengths of the WDM communications system for higher-speed communications has been developed. For example, a DWDM (dense wavelength division multiplexing) communications system has been devised as a WDM communications system. In the conventional WDM communications system, the number of multiplexed wavelengths is 32 or so while 100 or more wavelengths are being used in the DWDM communications system.
In the above mentioned optical communications system, it is necessary for a receiver to correctly receive an optical signal transmitted by a transmitter. In this connection, however, there arise some interfering factors by using an optical fiber. They are, for example, wavelength dispersion, a nonlinear effect, etc. To correctly receive a signal by a receiver, it is necessary that the Q value of the received signal is large, but the received waveform is degraded by wavelength dispersion, a nonlinear effect, etc., and the Q value often becomes unexpectedly small.
Therefore, to reserve an acceptable Q value, a conventional method of leveling the Q value in a receiving station has been used by adjusting the optical output of a transmitting station for each channel. Specifically, in the conventional technology, the improvement of the Q value is adjusted simply by increasing the optical output.
On the other hand, the deviation of the Q value is recently enlarged in the receiving station by an extension of a signal band (13 nm to 30 nm or more) and an increasing number of signal wavelengths (32 wavelengths to 100 or more). Accordingly, some channels are subject to degradation by nonlinearity. In the pre-emphasis (improving the Q value by increasing the optical output) in the conventional method used with the above mentioned system, the Q value of the channel of nonlinear deterioration is lowered.
Furthermore, an increasing number of signal wavelengths hardens the manual adjustment by an operator, and it is desired that the following conditions can be standardized to perform automatic control.
(1) a process of checking each channel for nonlinear deterioration.
(2) a method for determining the pre-emphasis when a channel is deteriorated.